Tonle Sap Lake is the largest freshwater lake in Southeast Asia. During the postmonsoon season, a small linear cloud system has been observed over this lake in early morning, while the sky above the surrounding land is clear. Although this cloud system is apparently generated by land breezes, previous studies on land-lake (sea) circulation have suggested that environmental factors at low latitudes inhibit development of nocturnal land breezes. In this study, we investigate the mechanism of these early morning clouds through numerical simulation. The simulations show a linear updraft system over the lake, forming along the southwest lakeshore around 22:00 and moving northeast to the middle of the lake. The heavier air mass from the land meets the extraordinarily warm and humid air mass over the lake, triggering updrafts under the conditionally convective instability. The characteristic high surface water temperature was favorable for generation of the land breeze and updraft systems. That high surface water temperature of the lake is produced by the tropical climate along with efficient energy absorption because of the shallowness of the water body. This unique feature can generate a clear nocturnal land breeze circulation accompanying a migrating updraft system over the lake despite its low latitude.
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